The present invention relates to an apparatus for separating gas fractions from a gas mixture having multiple gas fractions. In particular, the present invention relates to a gas separation system having a gas turbine for supplying feed gas to adsorbent beds at a number of discrete feed gas pressure levels for implementing a pressure swing adsorption process.
Gas separation by pressure swing adsorption (PSA) and vacuum pressure swing adsorption (vacuum-PSA) separate gas fractions from a gas mixture by coordinating pressure cycling and flow reversals over an adsorbent bed which preferentially adsorbs a more readily adsorbed component relative to a less readily adsorbed component of the mixture. The total pressure of the gas mixture in the adsorbent bed is elevated while the gas mixture is flowing through the adsorbent bed from a first end to a second end thereof, and is reduced while the gas mixture is flowing through the adsorbent from the second end back to the first end. As the gas separation cycle is repeated, the less readily adsorbed component is concentrated adjacent the second end of the adsorbent bed, while the more readily adsorbed component is concentrated adjacent the first end of the adsorbent bed. As a result, a xe2x80x9clightxe2x80x9d product (a gas fraction depleted in the more readily adsorbed component and enriched in the less readily adsorbed component) is delivered from the second end of the bed, and a xe2x80x9cheavyxe2x80x9d product (a gas fraction enriched in the more strongly adsorbed component) is exhausted from the first end of the bed. However, the conventional PSA and vacuum-PSA is deficient for several reasons.
Firstly, the conventional system for implementing PSA or vacuum-PSA uses two or more stationary adsorbent beds in parallel, with directional valving at each end of each adsorbent bed to connect the beds in alternating sequence to pressure sources and sinks. However, this system is often difficult and expensive to implement due to the complexity of the valving required.
Secondly, the conventional PSA or vacuum-PSA system makes inefficient use of applied energy, because feed gas pressurization is provided by a compressor whose delivery pressure is the highest pressure of the cycle. In PSA, energy expended in compressing the feed gas used for pressurization is then dissipated in throttling over valves over the instantaneous pressure difference between the adsorber and the high pressure supply. Similarly, in vacuum-PSA, where the lower pressure of the cycle is established by a vacuum pump exhausting gas at that pressure, energy is dissipated in throttling over valves during countercurrent blowdown of adsorbers whose pressure is being reduced. A further energy dissipation in both systems occurs in throttling of light reflux gas used for purge, equalization, cocurrent blowdown and product pressurization or backfill steps.
Energy efficiency has been improved in more modern PSA and vacuum-PSA systems, by using feed compressors (or blowers) whose delivery pressure follows the instantaneous pressure of an adsorber being pressurized, and by using vacuum pumps whose suction pressure follows the instantaneous pressure of an adsorber undergoing countercurrent blowdown. In effect, the feed compressor rides each adsorber in turn to pressurize it with reduced throttling losses, and likewise the vacuum pump rides each adsorber in turn to achieve countercurrent blowdown with reduced throttling losses. However, in such systems, each feed compressor can only supply gas to a single adsorber at any time, and each vacuum pump can only exhaust a single adsorber at a time. As a result, the working pressure in each such feed compressor or vacuum pump will undergo large variations, stressing the machinery and causing large fluctuations in overall power demand. Further, compression efficiency is compromised by the unsteady operating conditions.
Thirdly, since centrifugal or axial compression machinery cannot operate under such unsteady conditions, rotary positive displacement machines are typically used. However, such machines have lower efficiency than modern centrifugal compressors working under steady conditions, particularly for larger plant ratings (e.g. 50 tons per day oxygen vacuum-PSA systems). Further, scale up above single train plant capacities of about 80 tons per day oxygen is inhibited by the maximum capacity ratings of single rotary machines.
Lastly, the conventional system for extracting oxygen gas from air by pressure swing adsorption uses nitrogen-selective zeolites as the adsorbent material, such as Naxe2x80x94X, Caxe2x80x94X, and Caxe2x80x94A zeolites in the adsorbent beds. More recently, it has been found that low silica X zeolites (LSX) offer superior performance when exchanged with lithium (Lixe2x80x94LSX) or with lithium in combination with divalent or trivalent metal ions. It has also been found in the prior art that lithium exchanged chabazite performs well. However, to provide oxygen generation with favourable performance and efficiency, it has been necessary to conduct the pressure swing adsorption process over a relatively linear portion of the adsorber isotherm and over an operating range which is well below the nitrogen-uptake saturation point of the adsorbers. As a result, the conventional modern industrial tonnage oxygen separation system using lithium exchanged zeolites is operated at moderately sub-atmospheric pressures, requiring the use of expensive vacuum pump and compression machinery.
Accordingly, there remains a need for a gas separation system which is suitable for high volume and high frequency production, while reducing the losses associated with the prior art devices.
According to the present invention, there is provided a gas separation system which addresses the deficiencies of the prior art systems.
The gas separation system, according to the present invention, uses a pressure-swing adsorption process to separate a gas mixture into a first gas component of the gas mixture and a second gas component of the gas mixture. The gas separation system includes an adsorbent bed assembly having a number of flow paths for receiving adsorbent material therein for preferentially adsorbing the first gas component in response to increasing pressure in the flow paths in comparison to the second gas component. Each flow path includes a pair of opposite ends and a valve communicating with each opposite end for controlling a flow of the feed gas mixture through the flow paths. The gas separation system also includes axial or centrifugal compression machinery having a number of pressure inlet and outlet ports coupled to the valves for exposing each said flow path to a plurality of different pressure levels between an upper pressure and a lower pressure for separating the first gas component from the second gas component.
In a preferred embodiment of the invention, the adsorbent bed assembly includes a stator, and a rotor rotatably coupled to the stator. The stator has a pair of stator valve surfaces and a number of function compartments opening into the stator valve surfaces. The rotor includes a pair of rotor valve surfaces, each rotor valve surface being in communication with a respective one of the stator valve surfaces. The rotor also includes a number of flow paths for receiving adsorbent material therein. The ends of each flow path open into the rotor valve surfaces for communication with the function compartments.
The compression machinery comprises a gas turbine which includes a multi-stage compressor, a multi-stage expander, and a heat source. The compressor includes a number of outlet ports for delivering feed gas to feed gas compartments in the stator at a number of discrete pressure levels. The expander is coupled to the compressor and includes a number of inlet ports for receiving countercurrent blowdown gas from countercurrent blowdown compartments in the stator at a number of discrete pressure levels. A portion of the pressurized feed gas is fed from the compressor to the expander through the heat source so as to increase the speed of operation of the compressor and the expander. As a result, the output pressure and gas flow rate of the compressor is enhanced without resort to expensive electrical switch-gear, electric motors and step-up gearing.
In one implementation, the gas turbine uses a fuel combuster as the heat source, which can be supplied by low cost fuel gas frequently found at industrial sites where PSA oxygen generation or hydrogen purification is required. In another implementation, the compression machinery comprises a number of compressors, a number of expanders, and a heat source, with each compressor delivering feed gas to a feed gas compartment at a respective feed gas pressure level and each expander receiving blowdown gas from the blowdown compartments at a respective blowdown pressure level.
Each pressurization/blowdown compartment is in communication with typically several adsorbers being pressurized/unpressurized (in differing angular and time phase) at any given time. During pressurization and blowdown steps, the several adsorbers passing through each step will converge to the nominal pressure level of that step by a throttling pressure equalization from the pressure level of the previous step experienced by the adsorbers. Preferably the increments between adjacent pressure levels are sized to reduce irreversible throttling losses and to ensure that the gas flows entering or exiting the flow paths are substantially steady in both flow velocity and pressure. Further, gas flow is provided to the adsorbers in a pressurization step or withdrawn in a blowdown step at the nominal pressure level of that step. Hence flow and pressure pulsations seen by the gas turbine at each pressure level are minimal by averaging from the several adsorbers passing through the step, although each adsorber undergoes large cyclic changes of pressure and flow. As a result, the invention can attain favourable efficiency gains and capital cost economies of scale not previously attainable with gas separation systems employing centrifugal or axial compression machinery.
In one implementation of the invention, the adsorbers comprise nitrogen-selective adsorbents such as Caxe2x80x94X and Lixe2x80x94X. However, due to the high pressure output of the compression machine, the adsorbers are forced to operate at moderately elevated temperature. At high temperatures, saturation in nitrogen uptake is shifted to more elevated pressures, where isotherm nonlinearity is reduced. Therefore, preferably the adsorbers comprise Lixe2x80x94LSX, Caxe2x80x94LSX, Srxe2x80x94LSX, Znxe2x80x94LSX, Agxe2x80x94LSX, magnesium chabazite, calcium chabazite and strontium chabazite, and combinations thereof such as calcium/silver exchanged LSX.